Saturday, April 9, 2016

The Drogue Collection Grows

A few loaners round out the field...

 Delta Drogue 72 with Delta Drogue 24 on top.

Galerider 30. My favorite, at least at this point. Very smooth and easy to recover. But you need a huge size to generate serious braking (this one is too small for the PDQ--we needed a Galerider 36). I have a Seabrake 24 for that purpose--smaller, but twice the drag because it is solid.

... which led to more 1/3-scale models (not factory authorized, but as faithful as I could make them). Cone, Shark, Delta, Galerider, Seabrake.

Interestingly, the 1/3-scale models behave exactly like their big brothers. Drag is proportional to cross section, so the models product 9 times less drag. Drag ~ velocity^2 up to 14 knots, the limit of our testing. They also have very similar surfacing behavior, which lead to the premise that testing these with scale rode in Force 7 conditions, with steep waves and a little engine assist, is equivalent to gale conditions with a full scale drogue. Instead of loads approaching a ton, they stay below 150 pounds, and hand-over-hand recovery is 5-20 pounds instead of 50-200 pounds; makes a real difference when there are 100-300 feet of line out! The result is practical testing at more speeds and with more combinations, allowing full-scale testing to focus on the right questions.

After testing full scale drogues at speeds up to 7.5 knots and confirming scalablity, I built a universal cross reference table (Practical Sailor) so that you can pick a drogue for your boat, whether for steering or speed reduction. Generally it agrees with manufacturers, but they often have different theories regarding how much drag is best.

Now I am starting to focus on tandem drogues; does it make sense to trail 2 drogues on the same rode, as conditions get worse? The answer seems to be a resounding yes. Unlike anchors where rode forces destabilize anchors, in this case they seem to stabilize each other. Doesn't a second drogue add a lot of expense? Not really. Because stability is less of a problem, less expensive drogues like the Delta or Seabrake are valid. The rode and bridles are already paid for. Basically, you get the advantages of a series drogue, with more adjustability, flexibility, and easier handling.

The real bargain? It may be a surplus aircraft drogue I bought on E-Bay 15 years ago for $40. To my eye it is just as well built as the Paradrogue, and it gives a lot of stopping force for the buck. Too much for steering, too much for running a bar, but maybe just the thing for parking to drift fish.

Don't let anyone convince you that blocks work for making bridles. They just get stuck on the back of the boat, the obvious conclusion of a simple vector diagram. There are a lot of images in books and on the net that disregard the simple fact that the the block line must bisect the bridle angle.

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